Add support for auto-paginatable SearchResult type

[richardm-stripe]

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r? @ramon-stripe
cc @stripe/api-libraries

Summary

Adds support for autopaginatable SearchResult API response types. Implementation strategy: mostly I copied Collection classes/interfaces, but changed the pagination logic to use nextPage instead of lastId.

Analogous to PRs in stripe-node and stripe-go

See example commit for how this would work in the context of a /v1/search/charges endpoint.

Release Plan

Even though this functionality is not public, without a future PR adding a search method to a resource (like my charges example, none of these classes/interfaces are accessible, so in effect this addition does not affect the public interface of stripe-java and can be safely removed or changed. I intend to merge it right away. This is similar to the release pattern we followed for streaming requests.

Testing

Test strategy: took https://github.com/stripe/stripe-java/compare/richardm-search-charges?expand=1, made a .jar, added it to the pom.xml in my sample app, and successfully executed:

    RequestOptions opts = RequestOptions.builder().setStripeVersionOverride("2020-08-27;search_api_beta=v1").build();
    ChargeSearchParams params = ChargeSearchParams
      .builder()
      .setLimit(2l)
      .setQuery("metadata[\"foo\"]:\"bar\"")
      .build();
    for (Charge ch : Charge.search(params, opts).autoPagingIterable()) {
      System.out.println(ch.getId());
    }

and confirmed the pagination appeared to be happening correctly (it actually wasn't, at first and I fixed a bug).